The use of electronic devices to perform any number of tasks has steadily increased over time. This is especially true in the field of providing healthcare to patients. In the medical field, patient monitoring devices and/or systems are selectively coupled to a patient via at least one sensor, which senses information from the patient and is used in deriving at least one physiological parameter associated with the patient.
One type of patient monitoring device is a pulse oximeter. A pulse oximeter measures arterial blood oxygen saturation (SpO2) and pulse rate (PR) using principles of light transmission and absorption and generates a photoplethysmogram (PPG) signal. The pulse oximeter uses a sensor affixed to one of a predetermined position of the patient. Examples of predetermined positions on patients to which a pulse oximeter may be affixed includes, but is not limited to, a finger, foot, earlobe, toe, cheek, nose, nasal alar, scalp, wrist and torso. The sensor typically contains at least two light emitting diodes (LEDs) and a photodiode detector. The LEDs emit light at red (˜660 nm) and infrared (˜880 nm) wavelengths, some of which is absorbed by the patient's tissues and fluids, and some of which reaches the photodiode detector. The above described wavelengths of light emitted by the LEDs is described for purposes of example only and, in operation, the LEDs may emit light at any wavelength that falls within the red region and infrared region. More generally, it is possible the LEDs may emit light at two different wavelengths even outside the red and infrared regions. Furthermore, multi-LED sensors may measure at additional wavelengths outside the red and infrared regions. Oxygenated and deoxygenated hemoglobin absorb red and infrared light differently. Changing blood oxygen concentration changes the relative absorption at the two wavelengths. The acquired red and infrared signals can then be analyzed to measure the blood oxygenation. The greater the tissue and fluid between the emitters and detector, the less red and infrared light that reaches the detector. As a result, the measured PPG signals contain a constant (DC) component and a pulsatile (AC) component. The DC component results from the fixed absorbers, including skin, muscle, fat, bone, and venous blood. The AC component results from the periodic pulsations of the heart, driving changes in arterial blood volume. The PR can be measured from the PPG by detecting pulse peaks, and counting their number over a fixed time period (e.g., 60 s). The SpO2 can be measured by calculating the ratio of AC to DC components in both the red and infrared signals, which is commonly referred to as the “ratio of ratios” R and is illustrated below in Equation 1.R=(ACr/DCr)/(ACir/DCir)  (1)
The resulting value R determined in Equation 1 is used to look up the SpO2 value in an experimentally-determined reference table as is known in the art. Equation 1 is provided for the purposes of example only and, in operation, an oximeter may use a different method of calculating SpO2.
A drawback associated with PR and SpO2 values determined by the pulse oximeter is their susceptibility to noise present in the signal being measured. Types of noise that may be present in the signal being measured may include any of (a) electronic noise; (b) ambient light; (c) electrocautery noise and (d) any other type of noise from any source. The following illustrates an example where the noise present is electronic noise. However, it should not be construed to mean that the signal includes only a single type of noise. The signal may in fact include a plurality of different types of noise at any given time depending on the environmental conditions surrounding the pulse oximeter.
Analog electronic components introduce noise into the measured PPG signal. Noise corrupts the LED signal that is transmitted through the patient's tissue. When the patient's tissue is very opaque (e.g. when the sensor site is a thick appendage like a neonatal foot, or when the skin has dark pigmentation) most of the LED signal is absorbed in the tissue and only a weak signal is received by the oximeter circuit. The receiver circuit can compensate for a weak signal by amplifying it; however, the signal and noise are amplified together. To make matters worse, the act of amplifying the signal introduces additional noise. Indeed, most filtering and amplification operations performed in analog circuitry introduce additional noise into the measured signal. This decreases the signal-to-noise ratio (SNR). Thus, because the signal AC component is typically a very small fraction of the overall measured signal (often on the order of 1% or less), the AC signal may easily be obscured or overcome with noise resulting in an incorrect measurement of the AC component. This noise makes it more difficult for digital signal processing to estimate the PR and SpO2 because the AC values (ACr and ACir) used in Equation 1 would be less accurate. Incorrect measures lead to false alarms and contribute to the clinical problem of alarm fatigue, wherein clinicians become desensitized to overactive alarms. It is thus highly desirable to remove noise from the measured PPG signals, to improve patient monitoring.
The noise can be decreased and the SNR can be increased by narrowing the signal bandwidth. Some noise may be white (that is, constant across all frequencies), such as that introduced by resistive circuit elements. Other noise may have a 1/f distribution (that is, the noise becomes less powerful with increasing frequency), such as that introduced by active semiconductor circuit elements. In either case, the noise can be approximated as white since the PPG bandwidth of interest is very narrow, typically no more than ˜5 Hz. The power of white noise increases with the square root of the signal bandwidth. If the signal bandwidth can be reduced by a factor of four, the noise will be reduced by a factor of two.